1. Field of the Invention
The present invention relates to programmable cardiac pacing generators with telemetric capabilities.
2. Background of the Prior Art
Artificial cardiac pacemakers have been developed which have a large number of programmable features. For example, it has previously been possible to program the rate, the pulse width, the pulse amplitude, the refractory period, the sensitivity and the mode of operation of such pacemakers. The cardiac pacemaker could have its output inhibited and could respond to programming signals which would initiate a threshold margin test. Effects of closure of the control reed switch could be overridden, a hysteresis function could be employed, and it was also possible to provide for a high-rate limit which would exceed the normal upper rate limit of the pacemaker. Many of these programmable functions of the prior cardiac pacemakers could be programmed on the permanent or a temporary basis.
Prior programmable pulse generators further included means for signaling acceptance of a programming signal, and means to reset the program acceptance circuit if extraneous signals were detected as programming signals. The program signal acceptance circuit of the pacemaker performed several different checks on the detected programming signal including a parity check, an access code check, and a check to determine if a proper number of signals were transmitted within a given time period. Thus, it is seen that programmable cardiac pacemakers have reached a high level of sophistication.
Programming of implantable cardiac pacemakers is achieved by means of an external programming unit. Such pacemakers have RF receiving circuitry which is coupled to the transmitting-programming head of an external programming unit. The programming head of the external programming unit contained a magnet of a size which was sufficient to magnetically actuate a reed switch within the implantable cardiac pulse generator so as to close the reed switch when the programming head was properly positioned. The programming unit then programmed various functions of the cardiac pacemaker by sending a 32 binary digit (bit) word, with each bit being either a logic "1" or a logic "0" binary number. The signals generated by the programmer are typically burst of radio frequency signals at a frequency of approximately 175 KHz. For each word that is generated 33 virtually identical RF bursts of energy are applied. Pulse interval decoding is employed so that the real time separation between successive RF bursts determined whether or not these signals represent a logic "1" or a logic "0" bit.
In the programmer embodiment described herein, a relatively long time between bursts is defined as a logic "1" bit and a relatively short time is defined as a logic "0" bit. The pulse burst duration is on the order of approximately 0.35 milliseconds, the relatively long time between "1" logic bits is approximately 2.2 milliseconds and the relatively short time between "0" logic bits is approximately 1.0 milliseconds. Thus, an arbitrary series, (shown in FIG. 2) of 9 RF bursts defines a code of eight binary numbers. For the signal shown in the upper graph of FIG. 2, the binary code is "10010100". This binary number represents an octal number "224" in the conventional manner. The first number 2 of the octal number is represented by the first two most significant bits, the second number 2 of the octal number is represented by the next three bits, and the last number 4 of the octal number is represented by the last three significant bits.
The 32-bit words consist of four parts, each of which is a byte in length. These four parts are parameter code, data code, access code, and parity code and are generated in that order, least significant bit first. The first three bits of the eight-bit parameter code are not used whatsoever and are always generated as logic "0" bits. The fourth bit of the parameter code is either a logic "1" or a logic "0" bit, which respectively manifests either a temporary or permanent programming mode and the last four of the parameter bits represent the code for the particular function key that is depressed by the operator therein generating the program.
The data code pattern of the programming word consists of eight bits which define a particular value for the parameter selected.
Following the data portion of the programming word is the eight-bit access word which in the illustrated embodiment consists of the octal code "227". This word is utilized to start the process of programming the pulse generator. One purpose of the access code is to prevent extraneous signals which may be detected by a pulse generator from causing reprogramming. The final eight-bit portion of the programming word consists of an eight-bit parity code which is generated to provide proper parity based on the parameter and data portions of the word. Again, the parity portion is used as a check to prevent extraneous or undesirable programming of the pulse generator.
A number of patents have issued to assignee of the present invention which describe in detail the programmable cardiac pacemaker generally described above. These include U.S. Pat. Nos. 4,230,120, 4,233,985, 4,236,522, 4,241,736, 4,250,883, 4,253,466, 4,257,423, 4,263,915, 4,267,843, 4,273,132, 4,273,133, 4,275,738 and 4,276,883.
A programmer capable of programming the above described programmable cardiac pacemaker was developed which was capable of selecting the parameters and modes of operation of the pacing generator under the keyboard control of an operator. The construction and operation of a programmer constructed to program pacemakers constructed in accordance with the aforesaid patents is shown and described in detail in the following U.S. Pat. Nos. 4,236,524, 4,250,884, and 4,305,397.
With prior programmers of this type it is the practice to verify that the program that is transmitted to the pacemaker actually is accepted and is utilized to evoke a change in the programming of the pacemaker. To do this, skin electrodes are attached to the patient so that the skin electrodes provide artifact detection signals to the programmer. This signal consists of a program indicator pulse which has a 10% decrease in interval for one pacing cycle over the permanent rate pulses. This signal is detected by the skin electrodes, and the programmer indicates a "confirmed" message on its display. As an alternate to skin electrodes, EKG monitoring could be used with the prior programmer being constructed to pick up an artifact detection program confirmation indicating pulse. Thus, this prior programmer could provide programming signals which were inductively coupled from the programming head directly into the pacing pulse generator. However, detection of the operation of the pacing generator could only indirectly be confirmed by means of skin electrodes, or EKG, connections. Thus, while a program confirmation pulse could be received by these prior programmers through the skin electrodes, further confirmation that the correct programming change had been made by retransmission of the actual values of the parameters that were requested during the programming was not achieved in these systems.
As the battery voltage of the implanted pacemaker gradually drops, the pacemaker of the prior described patent will widen its pulse up to approximately 100% of its programmed value. In addition, the pulse rate of the threshold margin test will decrease from 100 ppm to 90 ppm, and rate hysteresis, if selected, will be inhibited. Prior to the programmer of the present invention, this rate change associated with the decrease of battery voltage could only be detected indirectly through means of skin electrodes, or EKG readings and a direct signal was not coupled from the pacing generator to the programmer to indicate battery condition.
The programmer of the present invention is constructed to function with SPECTRAX-SXT.TM. and ENERTRAX.TM. pulse generators manufactured by Medtronic, Inc., the assignee of the present invention. These pulse generators have output coils which are coupled to the programming head of the programmer, which function as transmitting coils. The programmer has receiving coils so that now the programmer can not only transmit, but can also receive telemetric information from the cardiac pacing pulse generators. The digital information that is stored in the pacing pulse generator can be transmitted in this manner includes:
(a) pulse generator model; PA0 (b) all permanent programmable parameters and values in effect; PA0 (c) whether the permanent pacing mode or the temporary "magnet" mode is in effect; and PA0 (d) an elective replacement time indicator if the battery voltage is dropped to its end-of-life level (EOL). This information will appear on the programmer display and will be printed if the printer is turned on. Connection to a EKG recorder or monitor, it is not necessary to obtain this information as it is with prior programming units. The programmer of the present invention therefore has an expanded capacity for handling digital information received from the programmed pacemaker.
Immediately following a permanent or a temporary programming transmission by the programmer, the program change will enter the pulse generator memory and is automatically retransmitted to the programmer via telemetry. This telemetric digital data is received by the programmer and compared with data stored in its own memory which is related to the preceding keyboard entries. If a data match occurs, the programmer will display a "CONFIRMED" message for display and printout. An elective replacement time ditial indicator signal will also be transmitted by the pulse generator if the battery has dropped to its EOL level. The extended capabilities of the programmer and programmed pulse generator thus allow not only for the receipt of the programmed confirmation indicator pulse, but also for verification that an exact match between the requested parameters and the parameters that have actually been programmed has occurred.
Although the expanded digital capabilities of the programmer of the present invention are of great importance, perhaps of even more importance is the fact that this programmer is able to receive and utilize telemetric analog information from programmable cardiac pulse generators such as the SPECTRAX-SXT.TM. and ENERTRAX.TM. pulse generators manufactured by Medtronic, Inc. Both the SPECTRAX-SXT.TM. and the ENERTRAX.TM. pulse generators can provide an intercardiac electrogram analog transmission (EGM). The patient, at the same time, may be connected to an EKG recording system and the EKG reading and the electrogram EGM reading may be compared by a digital processing system, thereby greatly increasing the diagnostic capabilities available to the operator of the programmer.
In addition to the intercardiac electrocardiogram, the ENERTRAX.TM. pulse generator also transmits function marker pulses. The function marker pulses of the ENERTRAX.TM. pulse generator can be recorded simultaneously with the surface EKG reading to allow visual observation of pulse generator operation by denoting both atrial and ventricular events. Each marker pulse is approximately 20 milliseconds wide and has a specific amplitude and polarity, which designates the following pulse generator functions: atrial sense, unused atrial, ventricular pace, ventricular sense, and ventricular refractory sense. A 60 millisecond wide error marker pulse may also be printed whenever the programmer receives interrupted or faulty marker coding from the pulse generator which is likely to occur in the presence of strong electromagnetic interference. Because of the variable polarity and amplitude of the marker pulse, the information transmitted relating to this type of marker information is essentially analog in nature. Prior to the SPECTRAX-SXT.TM. and the ENTERTRAX.TM. pacemakers, neither transmission of electrograms or transmission of marker pulses of this type was available.
A telemetry transmission system which made possible the transmission of both analog and digital data on the same output transmission channel is disclosed in U.S. Pat. No. 4,281,664 issued Aug. 4, 1981 to Stephen R. Duggan, and assigned to Medtronic, Inc. In this patent, either analog or digital information may be selectively applied to control the frequency of a voltage-controlled oscillator. For a digital voltage of a logic "1" level, the oscillator is at one frequency, and for a digital voltage of a logic "0" level, the oscillator is at a second frequency. Analog information, on the other hand, causes the frequency of the oscillator to vary about a nominal unmodulated frequency. Bursts of energy from the voltage control oscillator are controlled by a switching circuit which insures that the rate that the bursts are sent to a ringing tank circuit is controlled in accordance with the frequency of the oscillator. As the frequency of the oscillator increases the number of bursts increases and as the frequency of the oscillator decreases, correspondingly the number of bursts decreases.
This type of transmission system, as shown in the Duggan patent, may therefore be utilized to transmit either digital information relating to pulse width, pulse rate, etc., or the analog intercardiac electrogram transmission, or alternately a marker pulse transmission. In the case of the marker pulse transmissions, the particular sensed events such as ventricular pace, atrial sense, atrial refractory sense, ventricular sense, and ventricular refractory sense are digitally encoded and are applied to D/A circuit to generate an output signal of the appropriate polarity and signal level which causes the voltage-controlled oscillator to oscillate at a particular frequency above or below its nominal output frequency, in accordance with polarity and the magnitude of the marker pulse, for a predetermined period of time. This frequency identifies which of the marker pulses is being sent and a corresponding signal is converted into an appropriate marker pulse recording on a surface EKG record. The instantaneous frequency of the voltage-controlled oscillator that corresponds to the analog value of each marker pulse is translated into a pulse duration signal for a burst of energy which is transmitted to the programmer.